1. Technical Field
The disclosure relates to a structure of a semiconductor device. More particularly, the disclosure relates to a structure of thin film transistors.
2. Description of Related Art
When an active layer of a thin film transistor is made from a metal oxide semiconductor material, the tolerances of positive bias stress (PBS) and negative bias stress (NBS) are related to the amount of the oxygen vacancies in the metal oxide semiconductor material. When the metal oxide semiconductor material has more oxygen vacancies, the thin film transistor has better PBS tolerance, but poorer NBS tolerance. When the metal oxide semiconductor material has less oxygen vacancies, the thin film transistor has better NBS tolerance, but poorer PBS tolerance.
Since the thin film transistors in a display have various utilities and thus have various use conditions, some thin film transistors need better PBS tolerance, and some thin film transistors need better NBS tolerance. For example, in an organic light emitting diode display, each pixel needs two thin film transistors. One thin film transistor is a switch transistor, and the other thin film transistor is a driving transistor. The switch transistor stays in “off” state longer, and therefore the switch transistor need better NBS tolerance. The driving transistor stays in “on” state longer, and thus need better PBS tolerance. Accordingly, how to satisfy the various requirements of various transistors at the same time is a problem to be solved.